Emerging research has cast doubt on the popular paleo and keto diets, which emphasize high protein and low carbohydrate intake. While these dietary approaches have garnered a loyal following due to their short-term weight loss success, experts and recent studies are uncovering troubling health risks associated with their restrictive nature.
Keto’s rise and risks
The keto diet originated in the 1920s as a therapeutic tool for managing epilepsy, according to the National Library of Medicine. By severely limiting carbohydrates—often to fewer than 50 grams per day—keto forces the body into a metabolic state known as ketosis, where fat is burned instead of glucose for energy. While this can yield dramatic short-term weight loss, emerging evidence suggests significant long-term risks.
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A study published in the journal Frontiers in Nutrition analyzed more than 100 peer-reviewed papers on the keto diet and concluded that “the ketogenic diet is especially unsafe for pregnant women and people with kidney disease and may increase chronic disease risk for the general population.” The study linked the diet to elevated levels of low-density lipoprotein (LDL) cholesterol—commonly known as “bad” cholesterol—and an increased risk of cardiovascular disease, cancer, diabetes, and Alzheimer’s disease.
Vanessa Rissetto, a registered dietitian and co-founder of the nutrition company Culina Health, emphasizes the risks associated with carbohydrate restriction.
“Carbs are the main energy source for the body,” she told Fortune. “People on keto will likely be lacking fiber, minerals, and micronutrients, which impacts gut health, sleep, and energy—not to mention the potential overconsumption of red meat, which is directly linked to colon cancer and other health issues.”
The Cleveland Clinic warns of several adverse effects of the keto diet, including nutrient deficiencies, liver strain, kidney problems, constipation, and brain fog. While proponents argue that ketosis offers metabolic benefits, the diet’s high reliance on saturated fats has drawn criticism for its potential to harm cardiovascular health.
The paleo diet’s controversial premise
The paleo diet’s guiding philosophy stems from the idea that modern health issues result from the mismatch between contemporary diets and the foods consumed by our Paleolithic ancestors. Popularized by Loren Cordain’s 2001 book The Paleo Diet, like the keto diet, the approach emphasizes whole foods like lean meats, fish, fruits, vegetables, nuts, and seeds while excluding grains, legumes, dairy, and processed foods.
However, critics argue that the diet’s historical accuracy is flawed and its health claims are overstated. “It’s impossible to eat a true Paleolithic diet because food itself has changed so much—due to how we’ve changed the processing and farming of food and the evolution of plants,” said Federica Amati, a nutrition scientist at the health science company Zoe.
A study published in the journal Nutrients found that individuals following a paleo diet had higher levels of a blood biomarker associated with cardiovascular disease. While some studies have observed short-term benefits, such as weight loss and improved glucose control, long-term adherence to the diet raises concerns about nutrient deficiencies, particularly in calcium and vitamin D due to the exclusion of dairy products.
Comparing the two diets
Although keto and paleo differ in their origins and specific guidelines, both diets share a reliance on high-protein, low-carbohydrate frameworks. Keto emphasizes macronutrient ratios, with 60 percent of daily calories coming from fats, 30 percent from protein, and 10 percent from carbohydrates, according to the Cleveland Clinic. In contrast, paleo’s focus lies in avoiding processed foods and agricultural products like grains and legumes.
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This overlap in carbohydrate restriction aligns both diets against recommendations from the US Dietary Guidelines, which advise consuming 45 to 65 percent of daily calories from carbohydrates. Critics of these diets highlight that cutting carbs—a primary energy source—can lead to unintended health consequences, including fatigue, digestive issues, and poor gut health.
New research on plant-based proteins
Adding to the critique of keto and paleo diets, a recent study published in The American Journal of Clinical Nutrition examined the relationship between plant-based and animal-based protein intake and cardiovascular disease. Researchers analyzed data from three large prospective cohort studies: the Nurses’ Health Study, the Nurses’ Health Study II, and the Health Professionals Follow-Up Study, which collectively included over 200,000 participants.
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The findings revealed that higher plant-to-animal protein ratios were associated with a significantly lower risk of cardiovascular disease and coronary heart disease. Specifically, replacing just 3 percent of energy from animal protein with plant protein resulted in an 18-percent lower risk of cardiovascular disease and a 24-percent lower risk of coronary heart disease. However, the study found no significant impact on stroke risk.
During the study’s 30-year follow-up, the median plant-to-animal protein ratio increased from 0.36 to 0.50, reflecting a gradual shift toward plant-based eating. While the optimal ratio for preventing cardiovascular disease remains uncertain, the research underscores the benefits of incorporating more plant-derived proteins into the diet.
Environmental and ethical considerations
Beyond individual health impacts, keto and paleo have been criticized for their environmental toll. Both diets heavily rely on animal products, which are associated with higher greenhouse gas emissions, water usage, and land degradation compared to plant-based diets.
Amati highlights these concerns, stating, “They’re terrible for the environment and fairly unsustainable.”
A study by the University of Oxford supports this perspective, finding that shifting from animal-based diets to plant-based alternatives could reduce food-related greenhouse gas emissions by up to 73 percent. This aligns with the growing movement toward sustainable eating practices, which prioritize not only personal health but also the planet’s well-being.
Expert recommendations
Health professionals increasingly advocate for balanced and varied diets over restrictive approaches like keto and paleo. “To be in good health, you need a variety of food groups,” said Amati. “Everyone should have a personalized nutrition approach.”
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Rissetto echoes this sentiment, emphasizing the importance of moderation and gradual dietary shifts. “Go talk to a dietitian and have them walk you through this and work with you so you can have longevity … and maybe some sustainability around it,” she advises.
Instead of rigid dietary rules, experts recommend focusing on plant-forward eating patterns, which emphasize whole grains, legumes, fruits, vegetables, and healthy fats. Such diets are supported by extensive research, including studies linking the Mediterranean diet to reduced risks of heart disease, diabetes, and other chronic conditions.
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